A non-solvated form of [(Z)-O-methyl-N-(2-methylphenyl)thiocarbamato-κS](triphenylphosphane-κP)gold(I): crystal structure and Hirshfeld surface analysis

A near linear geometry for the gold(I) atom defined by a P, S donor set is found in the title compound; an intramolecular Au⋯O short contact is noted. Supramolecular layers sustained by C—H⋯π and π—π interactions feature in the crystal.


Chemical context
Triorganophosphanegold(I) carbonimidothioates, i.e. molecules of the general formula R 3 PAu[SC(OR 0 ) NR 00 for R, R 0 and R 00 = alkyl, aryl, were first described in 1993 as were the crystal and molecular structures of archetypal Ph 3 PAu[SC(OMe) NPh . Since then, approximately 70 crystal structures, including those of bidentate phosphanes and bipodal analogues, have been described in the crystallographic literature (Groom et al., 2016). The interest in phosphanegold(I) carbonimidothioates stems from two distinct considerations related to their relatively facile synthesis, their long-term stability and their readiness to crystallize, namely crystal engineering and evaluation for biological activity. In the former and reflecting their propensity to form diffraction-quality crystals, an unprecedented comprehensive series of compounds, R 3 PAu[SC(OMe) NC 6 H 4 NO 2 -p] (R = Et, Cy and Ph), and bidentate phosphane analogues, Ph 2 P-(CH 2 ) n -PPh 2 for n = 1-4 and for when the bridge is ferrocenyl, enabled correlations between the formation of AuÁ Á ÁAu (aurophilic) interactions and solid-state luminescence responses (Ho et al., 2006). In another series of compounds where the diphosphane ligand was held constant, i.e. [(Ph 2 P(CH 2 ) 4 PPh 2 ){AuSC(OR 0 ) NC 6 H 4 Y-p} 2 ] for R 0 = Me, Et or iPr and Y = H, NO 2 or Me, the packing was assessed in terms of delineating the influence of R 0 and Y substituents (Ho & Tiekink, 2007). In yet another systematic series of compounds, i.e. of the general formula R 3 PAu[SC(OMe) NR 00 ], for R = Ph, o-tol, m-tol or p-tol, and R 00 = Ph, o-tol, m-tol, p-tol or C 6 H 4 NO 2 -p, it was possible to assess the impact of steric and electronic effects upon the formation of intramolecular AuÁ Á ÁO or AuÁ Á Á(N-bound ring) interactions (Kuan et al., 2008). Over and above these studies, phosphanegold(I) carbonimidothioates exhibit promising biological potential in the context of anti-cancer activity (Yeo, Ooi et al., 2013;Ooi et al., 2015) and anti-microbial activity (Yeo, Sim et al., 2013). Just as systematic variations in the substituents influences the molecular packing, this also influences biological effects so that, for example, different apoptotic mechanisms of cell death are induced when the O-bound R 0 is varied. It was in fact during biological investigations that the title compound, Ph 3 PAu[SC(OMe) N(o-tol)] (I), was prepared once again, having been previously characterized as a 1:1 hemi-methanol solvate (IÁ0.5MeOH; Kuan et al., 2008). Herein, the crystal and molecular structures of (I) are described along with Hirshfeld surface analyses of both (I) and (IÁ0.5MeOH).

Structural commentary
The gold(I) atom in (I), Fig. 1, exists within the anticipated linear geometry defined by thiolate-S1 and phosphane-P1 atoms. Support for the 'thiolate-S1' assignment comes about by the elongation of the C1-S1 bond to 1.768 (3) Å , Table 1, c.f. 1.6700 (14) Å , and contraction of the C1-N1 bond in (I) to 1.260 (3) Å , c.f. 1.3350 (15) Å in the structure of the noncoordinating molecule, i.e. S C(OMe)N(H)(o-tol) (Kuan et al., 2005). The small deviation from linearity about the gold(I) atom [P-Au-S = 177.61 (2) ] may be related to the close approach of the O1 atom, AuÁ Á ÁO1 is 3.040 (2) , as the carbonimidothioate ligand is orientated to place the oxygen atom in close proximity to the gold atom, Fig. 1. There are also significant differences in key angles between the coordinating and non-coordinating forms of the ligand, especially about the C1 atom. These reflect the reorganization of -electron density manifested in the C N and C S bonds, respectively. Thus, the widest angles in the anion involve C N and those in the free molecule, involve C S. A relatively large change is noted for the C1-N1-C2 angles, i.e. 121.4 (2) and 127.11 (12) , respectively, for the coordinating and non-coordinating ligands, which is a result of the presence of the acidic proton in the latter. In terms of conformation of the anion in (I), the central residue comprising the S1, O1, N1 and C1 atom is strictly planar (r.m.s. deviation of the fitted atoms = 0.0091 Å ), with the pendent C2 and C9 atoms lying 0.035 (4) and 0.198 (4) Å out of this plane, respectively. The dihedral angle between the central residue and the N-bound aryl ring is 85.08 (7) , indicating a nearly perpendicular arrangement; in the free ligand the comparable angle is 51.84 (6) (Kuan et al., 2005). Salient geometric parameters for (IÁ0.5MeOH) (Kuan et al., 2008) are also included in Table 1. From these data, it is apparent there are no great variations between the structures with perhaps the exception of the Au-S1 bond length in (I) being 0.01 Å longer than in (IÁ0.5MeOH). In terms of angles, the angle subtended at the S1 atom is about 2 tighter in (I). The intramolecular AuÁ Á ÁO1 separation is 0.05 Å shorter in (I) but the deviation from linearity is less, reflecting the weak nature of this interaction. Molecular structure of (I), showing the atom-labelling scheme and displacement ellipsoids at the 70% probability level. 3.040 (2) 3.093 (5) S1-  177.61 (2) 175.52 (6) Au-S1-C1 103.14 (9) 105.0 (2) C1-O1-C9 114.9 (2) 116.3 (5) C1-N1-C2 121.4 (2) 121.2 (6) S1-C1-O1 113.38 (18) 113.5 (4) S1-C1-N1 125.9 (2) 126.0 (6) O1-C1-N1 120.7 (2) 120.5 (6) Note: (a) Kuan et al. (2008). Fig. 2 shows an overlay diagram for (I) and I in (IÁ0.5MeOH). From this it can be seen there is evidently a close overlap of all but the aryl rings that display orientational differences.

Supramolecular features
In the crystal of (I), the most prominent points of contact between molecules are of the type C-HÁ Á Á and -, Table 2. Thus, centrosymmetrically related o-tolyl residues associate via pairs of methyl-C-HÁ Á Á(o-tol) interactions, and centrosymmetrically related phosphane ligands are connected via face-to-faceinteractions involving one of the P-bound phenyl rings only. The result is the formation of supramolecular layers lying parallel to (011) as illustrated in Fig. 3a. The layers stack with no directional interactions between them, Fig. 3b.
The packing of (IÁ0.5MeOH) is also characterized by supramolecular layers. These are sustained byinteractions of 3.687 (4) Å between centrosymmetrically related molecules in a face-to-face fashion, as for (I), and by phenyl- Overlay diagram of (I) (red image) and I in (IÁ0.5MeOH) (blue). The molecules have been overlapped so that the S1, O1 and N1 atoms are coincident. Table 2 Hydrogen-bond geometry (Å , ).

Figure 3
Molecular packing in (I): (a) a view of the supramolecular layer sustained by C-HÁ Á Á andcontacts, shown as purple and orange dashed lines, respectively, and (b) a view of the unit-cell contents shown in projection down the a axis, highlighting the stacking of (011) layers.

Figure 4
Molecular packing in (IÁ0.5MeOH): a view of the unit-cell contents shown in projection down the a axis. The C-HÁ Á ÁS, C-HÁ Á Á andcontacts are shown as orange, purple and blue dashed lines, respectively. The methanol molecules are highlighted in space-filling mode.
successive layers. This arrangement defines columns along the a axis in which reside the disordered methanol molecules, Fig. 4. The partially occupied methanol molecules in (IÁ0.5MeOH), disordered over a centre of inversion, are connected to the host framework via methyl-C-HÁ Á ÁS interactions.

Analysis of the Hirshfeld surfaces
Hirshfeld surface analysis and fingerprint plots were undertaken to study the intermolecular contacts and topological differences between (I) and its methanol hemi-solvate, (IÁ0.5MeOH). Briefly, the internal (d i ) and external (d e ) distances of atomic surface points to the nearest nucleus were computed for the molecules in both (I) and (IÁ0.5MeOH) (Spackman & Jayatilaka, 2009;McKinnon et al., 2007). The resulting normalized contact distances (d norm ) were mapped on the Hirshfeld surface in the range À1.04 to 1.91 Å . The contact distances shorter than the sum of van der Waals radii are highlighted in red while distances equal to or longer than the sum of van der Waals radii are shown in white and blue, respectively (McKinnon et al., 2007). The combination of d i and d e in intervals of 0.01 Å result in the two-dimensional fingerprint plots, where the different colours on the fingerprint plots represent the probability of occurrence, ranging from blue (few points) through green to red (many points) (Spackman & McKinnon, 2002). All analyses were performed using Crystal Explorer (Wolff et al., 2012). The number of Hirshfeld surfaces that are unique in a given crystal structure depends on the number of independent molecules in the asymmetric unit (Fabbiani et al., 2007). For this reason, the Hirshfeld surfaces for (IÁ0.5MeOH) were modelled separately for (I) and for MeOH, while the Hirshfeld surface of (IÁ0.5MeOH), as a whole, were also included for a thorough comparison of the molecular packing in (I) and (IÁ0.5MeOH). Fig. 5a and 5b show the front and back views of Hirshfeld surfaces for (I), (IÁ0.5MeOH) as well as for I in (IÁ0.5MeOH) which are displayed in approximately the same orientation. Despite the presence of additional solvent molecule in (IÁ0.5MeOH), both this and (I) are governed by similar intermolecular contacts as can be observed through the appearance of several red spots on the Hirshfeld surfaces of both structures. These are mainly attributed to HÁ Á ÁH, CÁ Á ÁH/ HÁ Á ÁC and SÁ Á ÁH/HÁ Á ÁS contacts. However, a close inspection of the Hirshfeld surface of I in (IÁ0.5MeOH) reveals a stark difference as compared to (I), in that evidence is found for a close contact through a SÁ Á ÁH interaction with the solvent MeOH molecule as readily seen from the intense red spot in Fig. 5a -right. Apart from this contact, I in (IÁ0.5MeOH) also forms weak interaction, as demonstrated by the less intense red spot in Fig. 5b -right, through OÁ Á ÁH with another molecule of I but beyond the sum of their van der Waals radii (Spek, 2009).
In view that the conformational flexibility highlighted in Fig. 2, the mapping of curvedness over the Hirshfeld surface was undertaken in order to correlate these with some physicochemical properties. Fig. 5c and 5d show the front and back views of the curvedness for (I), (IÁ0.5MeOH) and I in (IÁ0.5MeOH). From these views, it is clear (I) exhibits a relatively broad region of curvedness surface, Fig. 5c -left. It is presumably for this reason that (I) has a relatively greater surface area, indicating a more compact conformation, i.e. having a lower volume, and is more densely packed than I in (IÁ0.5MeOH), see data in Table 3. Interestingly, it seems the molecular shape exerts a great influence over the intermolecular interactions and the density of the resultant crystal structures, Table 3. The packing efficiency of (I) is also greater than that of (IÁ0.5MeOH), suggesting that the incorporation of research communications   methanol in the molecular packing of (IÁ0.5MeOH) is not directed by the need to fill otherwise free space in (I).
The complete two-dimensional fingerprint plots for (I), (IÁ0.5MeOH) and, for additional comparison, I in (IÁ0.5MeOH), along with the decomposed two-dimensional plots for the indicated interactions are presented in Fig. 6, while the percentage contributions are represented graphically in Fig. 7. As mentioned previously, molecules of (I) in its unsolvated and solvated forms are governed by similar intermolecular close contacts which mainly comprise nonhydrogen-bond interactions. Specifically, HÁ Á ÁH, being the most dominant interaction among all, ca 57.3% in (I) and 55.4% in (IÁ0.5MeOH), forms a forceps-like fingerprint in (I), by contrast to the distinctive spike of (IÁ0.5MeOH), Fig. 6b. It is noted there is not much to distinguish the fingerprint patterns due to CÁ Á ÁH/HÁ Á ÁC, Fig. 6c. This observation is vindicated by the near equivalence of the sums of the d e + d i distances of $2.70 Å for (I) and $2.64 Å for (IÁ0.5MeOH) and with the relative contributions of approximately 23.3 and 23.8% to the overall surface areas, respectively. However, a marked difference is observed in the corresponding pincerslike fingerprint plots due to SÁ Á ÁH/HÁ Á ÁS interactions, Fig. 6d. Thus, the plot for (I) displays a sum of intermolecular contact distance d e + d i of $2.88 Å , originating from weak phenyl-C-HÁ Á ÁS contacts. For the solvate, a mixed interaction mode is evident from the asymmetric fingerprint plot indicating interactions between two chemically and crystallographically distinct molecules, i.e. the relatively strong solventÁ Á Ásolute methyl-C-HÁ Á ÁS interaction with the sum of d e + d i distances being $2.42 Å coupled with a weak methoxy-C-HÁ Á ÁS contact with d e + d i = $3.1 Å . Such interactions contribute roughly 3.2% (SÁ Á ÁH-solvent) and 1.1% (SÁ Á ÁH-methoxy) to the total 4.3% to the overall Hirshfeld surface of I in (IÁ0.5MeOH) compared to a $7.5% contribution in (I). Molecule (I) does not forms any meaningful contacts through OÁ Á ÁH/HÁ Á ÁO owing to their long contact distances despite these contacts constituting approximately 2.4% of the overall contacts on the Hirshfeld surface, Fig. 6e. Upon crystallization with methanol solvent, the overall contribution increases to 6.4% with the sum of d e + d i of $2.50 Å which is considered longer than typical OÁ Á ÁH interactions with distances of $2.14 Å (Gavezzotti, 2016).

Database survey
As mentioned in the Chemical context, there are over 70 molecular structures in the crystallographic literature (Groom et al., 2016) based on the general formula R 3 PAu[SC(OR 0 ) NR 00 for R, R 0 and R 00 = alkyl, aryl. The present structural pair, (I) and (IÁ0.5MeOH) represents the second example of solvatomorphism, with the prototype compound Ph 3 PAu[SC(OMe) NPh  being also found in a chloroform solvate (Kuan et al., 2008). The common feature of all four molecules is the presence of intramolecular AuÁ Á ÁO interactions. Very recently, a poly-   Comparison between (I), (IÁ0.5MeOH) and I in (IÁ0.5MeOH) of (a) the full fingerprint plots, and delineated two-dimensional plots associated with (b) HÁ Á ÁH, (c) CÁ Á ÁH/HÁ Á ÁC, (d) SÁ Á ÁH/HÁ Á ÁS and (e) OÁ Á ÁH/HÁ Á ÁO contacts. morph of Ph 3 PAu[SC(OEt) NPh has been reported (Yeo et al., 2016) in which there has been a dramatic conformational change compared with the previously described form . While the latter features the normally observed AuÁ Á ÁO interaction, the new form features intramolecular AuÁ Á Á (Caracelli et al., 2013) interactions. It was suggested that the crystallization conditions determined the conformation with that featuring the AuÁ Á Á interactions being the thermodynamic outcome (Yeo et al., , 2016.

Synthesis and crystallization
IR spectra were obtained on a Perkin-Elmer Spectrum 400 FT Mid-IR/Far-IR spectrophotometer from 4000 to 400 cm À1 ; abbreviation: s, strong. The 1 H NMR spectrum was recorded in CDCl 3 on a Bruker Avance 400 MHz NMR spectrometer with chemical shifts relative to tetramethylsilane; abbreviations for NMR assignments: s, singlet; d, doublet; t, triplet; m, multiplet.

[(Z)-O-Methyl-N-(2-methylphenyl)thiocarbamato-κS](triphenylphosphane-κP)gold(I)
Crystal data [Au(C 9  Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.